Solar Storm of September, 1859 — known as the Carrington Event; European and North American telegraphs failed, some shocking their operators; transmission towers gave off visible sparks, and some systems could continue to send and receive messages even with a severed power supply connection.
Aurora of November, 1882 — telegraph systems rendered useless.
Geomagnetic Storm of May, 1921 — northern lights appeared in much of the eastern United States; telegraph service knocked out due to blown fuses.
Geomagnetic Storm of May, 1967 — sun fired off a flare so powerful that it was visible to the naked eye, and began emitting radio waves at a level that had never been seen before; US Air Force early nuclear warning radars fails; leaders assume the Soviets were jamming and possibly preparing a preemptive strike.
Geomagnetic Storm of March, 1989 — aurora seen in Texas and Florida; satellites in polar orbits became uncontrollable for several hours; Quebec suffered a major blackout.
Bastille Day Solar Flare of July, 2000 — one of the most violent sun storms in recorded history; X-class flare, the highest designation possible; disrupted satellites and blocked radio comms.
Halloween Solar Storms of 2003 — satellite-based systems and comms affected; aircraft near the polar regions were advised to avoid high altitudes; Sweden suffered a blackout.

Within approximately 8 minutes of a solar storm, radiation crashes into our atmosphere, hampering radio signals. The power of the radiation coming from the sun is calculated in an index called the "flux density." That number is typically 100. It could be 300 during the peak of a solar cycle. That number could reach into the several thousands during a major solar storm. Historic super storms have caused swirling northern lights to appear in the sky as far south as New Mexico. Texas and Florida.

Scientists cannot predict them. Modelling is rudimentary. The Government learns about them only when the photons arrive. Disruptions to high-frequency radios, GPS, and other basic communications tools are expected. CMEs are bands of particles that often follow solar flares. They can take down unprepared power grids indefinitely. Current monitoring gives authorities about 30 minutes warning before the fastest, most dangerous CMEs strike.

The occurrence of this Indicator is tightly associated with the Grid Down indicator. However, interruptions to military systems increase the fog of war or pre-war.